"That is right; and the most common form of combustion is the combination of carbon with oxygen. This is commonly employed, not because it generates the most intense heat, but because carbon exists so abundantly, and is the most available and the cheapest. The most common form of carbon is wood and coal. This is that storehouse of heat which God has placed in man's keeping. Without this the larger part of the earth's surface would be uninhabitable. This renders culture and civilization possible. Without it the arts could have no existence. The key of this storehouse of heat God has given to man, so that he may enter in and use its treasures at his pleasure. In the finer arts where very great heat is required, hydrogen is used in place of carbon. Jets of oxygen and hydrogen gas thrown together constitute what is called the oxy-hydrogen blowpipe, and generate the intensest heat which can be produced by man. "Another source of heat not often mentioned is electrical force. This, like mechanical force, may be transmuted into heat. An electric current sent through an insufficient or poor conductor heats it, and, if the current be sufficiently strong, consumes it. Thus lightning-rods are sometimes melted and buildings set on fire. "These, then, are the natural reservoirs of heat: 1, the sun and other heavenly bodies; 2, the internal heat of the earth; 3, living force, or motion; 4, chemical action; 5, electric force. "We can hardly over-estimate the abundance of these natural supplies of heat. The world is warmed on the most munificent scale. The earth receives from the sun heat sufficient to boil three hundred cubic miles of ice water per hour, and the whole sum of the sun's heat would boil 700,000,000,000 cubic miles of ice water in the same time, that is, the heat radiated by the sun would boil a mass of ice water of the size of our globe in twenty-five minutes. "The amount of carbon provided by the Creator is enormous beyond conception. Vast regions of country are covered with dense forests, but the fuel from the forests is but a handful in comparison with the fuel stored up in coal-beds below the surface of the earth. Mr. Mitchel estimated the extent of the coal-beds of a portion of Europe as follows: Great Britain, 12,000 square miles; Spain, 3500; France, 1700; Belgium, 5180. Mr. R. C. Taylor has made a like estimate for North America, giving to British America 18,000 and to the United States 134,000 square miles. "These estimates, you will notice, say nothing of Asia, Africa, South America, or the islands of the sea, and include only the smaller part of Europe. In the United States, also, new coal-fields are constantly discovered. The supply of carbon for fuel seems exhaustless. In the British islands about 100,000,000 tons of coal are mined annually. At this rate the known supply would last for a thousand years. In the United States the supply has no known limit.